Chain surveying – setting out perpendiculars: Which instrument is commonly used to set out perpendicular offsets to a chain line during chain surveying?

Introduction / Context:
Short offsets at right angles to a baseline are routine in chain surveying for locating details like boundaries, hedges, and walls. A simple, quick instrument is preferred over high-precision angle-measuring devices for efficiency in the field.

Given Data / Assumptions:

• The task is to set out a 90° offset from an established chain line.
• Speed and simplicity are more important than arc-second precision.
• Visibility is adequate for sighting the detail point.

Concept / Approach:
The optical square uses a fixed 45° prism or mirror system to create a right angle between two lines of sight, enabling a rapid and reasonably accurate setting of perpendiculars. While a theodolite can set any angle, it is unnecessarily elaborate for common offsets. A prismatic compass is for bearings, not precise perpendiculars, and a levelling instrument measures vertical differences, not plan angles.

Step-by-Step Solution:

Verification / Alternative check:
Cross-check by measuring a 3-4-5 triangle or by setting a right angle with a cross-staff if available; the points should coincide within field tolerances.

Why Other Options Are Wrong:

• Theodolite is accurate but slow for routine offsets.
• Prismatic compass is affected by local attraction and is not intended for right angles.
• Levels determine elevation, not plan angles.

Common Pitfalls:
Using the instrument far from the chain line point; parallax errors; ignoring slope corrections when offsets are long on sloping ground.

Final Answer:
Optical square